1. Field of the Invention
The present invention relates to an exposure apparatus usable in manufacturing a device, such as a semiconductor chip (e.g., integrated circuit (IC) or large scale integrated circuit (LSI)), a liquid crystal panel, a charge coupled device (CCD), a thin-film magnetic head, or a micro machine.
2. Description of the Related Art
A lithography manufacturing method is publicly known as a method for forming a desired circuit pattern on a semiconductor. The lithography includes a process of exposing a semiconductor substrate, on which a photosensitive organic film (photo resist) is coated, to light via a mask (reticle), on which a circuit pattern is formed.
Recent highly-integrated LSI requires more refinement of a circuit pattern. To realize this requirement, the above-described lithography requires improvement in a resolving power of an exposure apparatus that performs exposure processing.
As defined in the following formula, the resolving power of an exposure apparatus is proportional to a wavelength λ of a light source and is inverse proportional to a numeric aperture (NA) of a projection optical system where k1 represents a proportional constant.Resolving power=k1·(λ/NA)  (1)Accordingly, to improve the resolving power of an exposure apparatus, it is useful to decrease the wavelength of a light source or increase the numeric aperture of the projection optical system.
The depth of focus (DOF) is one of the optical characteristics of an exposure apparatus. The depth of focus is a distance from a focal point that represents an allowable defocus range of a projected image. The following formula expresses the depth of focus, wherein k2 represents a proportional constant.DOF=k2·(λ/NA2)  (2)Accordingly, if the wavelength of a light source is reduced or the numeric aperture of a lens is increased to improve the resolving power of an exposure apparatus, the depth of focus decreases. As a result, the defocus state of a projected image may be out of the allowable range.
Especially, realization of a fine and solid circuit pattern is one of key factors for a next-generation device that is expected to attain high integration. Thus, reduction in the depth of focus is a serious problem. Namely, a solid circuit pattern requires a relatively long processing dimension in the optical-axis direction. Therefore, a large depth of focus is required. A sufficient depth of focus is constantly required regardless of the fineness of a circuit.
To solve the above-described problem, an attempt to enlarge the depth of focus may be performed by projecting a mask pattern on a substrate using exposure light having plural wavelengths so as to form images on different positions on the optical axis.
For example, as discussed in Japanese Patent No. 2619473, a conventional optical system includes a light source configured to generate light oscillating at a first wavelength, a light source configured to generate light oscillating at a second wavelength, and a unit configured to generate composite exposure light obtained from two light sources.
Furthermore, as discussed in Japanese Patent Application Laid-Open No. 11-162824, a conventional optical system includes a filter provided on an optical path between a light source and a wafer that can selectively transmit plural wavelength bands of light, to perform an exposure operation using exposure light having plural wavelengths. The above-described two conventional systems enable the exposure light to have plural wavelength spectra.
The amount of exposure (i.e., dose) during a wafer exposure operation is accumulated for a predetermined number of pulsed light emissions (or during a constant period of time) As discussed in Japanese Patent Application Laid-Open No. 06-252021, a setting wavelength of a light source can be changed during a wafer exposure operation so as to realize an exposure of a wafer using light having plural wavelengths. According to this conventional system, the wavelength of pulsed light emitted during an exposure operation is controlled in a range from −Δλ to +Δλ as illustrated in FIG. 5. The cumulative spectrum distribution on a wafer becomes a spectrum having two peak wavelengths as illustrated in FIG. 4.
FIG. 6 illustrates an excimer laser usable as a light source of a general exposure apparatus. The excimer laser includes a laser chamber 27 filled with a laser gas. A pair of main discharge electrodes 30 and 31 is provided in the laser chamber 27. The main discharge electrodes 30 and 31 generate a discharge that can excite the laser gas filled in the laser chamber 27 and generate light in the laser chamber 27.
The generated light can be amplified while it passes through windows 28 and 29 and oscillates between a front mirror 32 and a narrow-banding unit 40. A wavelength selection element (e.g., a prism or a grating) provided in the narrow-banding unit 40 outputs narrow-band light as a laser beam 33 from the front mirror 32. Part of the output laser beam 33 reflects on a beam splitter 34 and enters a monitor etalon 36 (wavelength monitor) and a diffraction grating spectroscope 37. The monitor etalon 36 and the diffraction grating spectroscope 37 measure an oscillation central wavelength λcr and an oscillation spectrum width Δλ of the output laser beam 33. A wavelength controller 38 receives the measurement values from the monitor etalon 36 and the diffraction grating spectroscope 37.
When the above-described excimer laser controls its setting wavelength as illustrated in FIG. 5 to perform an exposure operation while maintaining the energy intensity at the same level, the symmetric wavelength spectrum illustrated FIG. 4 may not be obtained. The obtained wavelength spectrum may be asymmetric as illustrated in FIG. 7.
This phenomenon depends on the characteristics of an optical system of the excimer laser. For example, the reflectance of a grating (i.e., wavelength selection element) varies according to a selected wavelength. Therefore, the light intensity at −Δλ (i.e., a wavelength shorter than a central wavelength having been set) differs from the light intensity at +Δλ (i.e., a wavelength longer than the central wavelength having been set). A difference between the light intensity at −Δλ and the light intensity at +Δλ causes a defocus.